The World of Cells

The cell is the basic unit of life. Every organism is either made up of large interconnected groups of cells or is itself a cell, free-floating and independent. Naturally, given the diversity of life that exists on earth, there is a nearly endless variety of cell types. The human body alone contains more than two hundred different kinds of cells that vary in size, shape, and function.

Biologists generally classify all cells into two different categories, depending on the presence or absence of a nucleus. Cells that lack a nucleus are called prokaryotes; cells that have a nucleus are called eukaryotes. The majority of prokaryotes are bacteria. The DNA of these cells is contained in a single chromosome that floats freely in the cell's cytoplasm. Nearly all other cells, including all plant and animals cells, are eukaryotic, and thus keep most, if not all, of their genetic material in a nucleus.

Cells of different types vary widely in size. Generally, prokaryotes, which range from 1 to 10 microns (thousandths of a millimeter) in diameter, are smaller than eukaryotes. One E. coli bacterium, for example, measures only about two microns in diameter, even smaller than a single mitochondrion, a eukaryotic organelle. Most eukaryotic cells average about 20 microns, while some cellular giants, including human egg cells, measure 100 or more microns across, the diameter of a typical human hair.

At first glance, the shape of most cells is fairly ordinary. Many animal cells are spherical; plant cells are typically boxy. But if you look more carefully, you'll see that a cell's shape often relates closely to its role in the organism or its behavior in its environment. The outermost cells in your skin, for example, are flattened and overlap each other like shingles on a roof. And like a roof, your skin protects your body from the environment. Neurons are even more specialized. Like most cells, neurons have a spherical cell body with a nucleus inside. But neurons also have threadlike projections, called dendrites and axons (some axons are three feet long!), that transmit nerve impulses throughout the body.

Genetic information contained in the DNA in every one of our cells tells those cells how to behave by dictating which proteins cells should create and when cells should create them. And amazingly, thanks to the process of DNA replication, all of this detailed information can be passed not only from parent to offspring but from "parent cell" to "daughter cell" whenever the parent cell divides.

PART I
Take time to look at the pictures below and describe each picture and what their shape might suggest about the cells function.

Cell Gallery

After viewing the above cell pictures answer the following two questions. Take time to really investigate these pictures in order to formulate a meaningful answer to the questions.

Discussion Questions

What would you expect the interior of a paramecium to look like?
What can you infer about function from the structure of any of these cells?

PART II
Watch the following video "Nucleus, Cytoplasm, and Membrane"

Discussion Questions

Why do you think that cells producing large numbers of proteins have a greater number of rough endoplasmic reticula than those producing fewer proteins?
How does cell structure suggests cell function?
How are substances and structures transported within the cell?

PART III
Explore the Cell Membrane: Just Passing Through Web activity. Discuss the following:

Discussion Questions

How does the composition of the cell membrane help regulate the movement of substances into and out of the cell?
Describe the different processes involved in the movement of these substances into and out of the cell: oxygen, carbon dioxide, glucose, potassium, sodium, water, and enzymes

PART IV
Watch the Organelles in the Cytoplasm video.

Discussion Questions

What is meant by the phrase "the cell is the functional unit of life"?
How are the organelles in a cell like the organs in a human body?
Why do you think cells that produce large numbers of proteins have more rough endoplasmic reticulum (ER) than cells that produce fewer proteins?
In which kinds of human cells would you expect to find the most mitochondria? The most lysosomes? The most ribosomes? Explain your answers.

PART V
Examine the Animal and Plant Cell stills and identify the similarities and differences between the two types of cells. Ask:

Discussion Questions

What is the most significant difference between animal and plant cells? Why do you think that?